1. Field of the Invention
The present invention relates to methods of manufacturing the external connections of a circuit board, and to methods of manufacturing a semiconductor device package comprising a circuit board having such external connections. More particularly, the present invention relates to methods of manufacturing a circuit board having metal bumps as its external connections, and to methods of manufacturing a semiconductor device package comprising the same.
2. Description of the Related Art
Semiconductor device packages are progressively becoming faster, smaller, and thinner in order to meet the pressing demands for the miniaturization and multi-functionalization of electronic apparatus. A Ball Grid Array Package (hereinafter, referred to as a `BGA package`) has been developed in connection with these trends. The BGA package is a type of surface mount package which uses a printed circuit board (PCB) and solder balls or solder bumps, instead of a lead frame, for electrically connecting the semiconductor chip and the main circuit board. The BGA package has a comparatively large number of the I/O pins, and thus possesses a high mounting density.
As mentioned above, the BGA package has a structure in which the semiconductor chip is attached and electrically connected to the PCB. Also, circuit wiring patterns formed on a surface of the PCB, to which the semiconductor chip is attached, are electrically connected to external connections formed on the other surface of the PCB through a plurality of via holes. Because the external connections are not formed on the surface to which the semiconductor chip is attached, the BGA package can have a mounting area, i.e., an area over which the BGA package is mounted to the main board, that is smaller than that of other conventional plastic packages. In conventional BGA packages, solder bumps are used as the external connections.
FIG. 1 depicts such a conventional BGA package. The BGA package 200 includes a semiconductor chip 130 which is electrically connected to solder bumps 128. A PCB 110 is positioned between the semiconductor chip 130 and the solder bumps 128 and is used as a means for connecting the semiconductor chip 130 and the solder bumps 128.
A copper (Cu) pattern layer is formed on upper and lower surfaces of the PCB 110 so as to facilitate the electrical connection of the semiconductor chip 130 to the solder bumps 128. A plurality of via holes 124 are formed in the PCB 110 in order to interconnect the Cu pattern layers which are deposited on the upper and the lower surfaces of the PCB 110. In addition, Cu forms the inner walls of the via holes 124.
The Cu pattern layer on the upper surface of the PCB 110 forms a chip mounting area 132 and circuit patterns 123. The chip mounting area 132 is the region on which the semiconductor chip 130 will be mounted. The respective circuit patterns 123 are positioned around the chip mounting area 132. One end of each circuit pattern 123 serves as a wire bonding area 125 which is electrically connected to the semiconductor chip 130 by a bonding wire 134.
The Cu pattern layer on the lower surface of the PCB 110 consists of a plurality of solder ball pads 126. The solder ball pads 126 are made of a metal, and solder balls will be attached thereto. The via holes 127 which are formed below the chip mounting area 132 are for transferring the heat generated during the operation of the semiconductor chip 130 to the outside. Hereinafter, these via holes 127 will be referred to as `the emission via holes` 127.
Before electrically connecting the semiconductor chip 130 to the PCB 110 with the bonding wires 134, the upper and the lower surfaces of the PCB 110 are coated with solder resist 120. The solder resist 120 is applied over all portions of the upper and lower surfaces except for the chip mounting area 132, the wire bonding area 125, and the area of the solder ball pads 126. After that, the upper surface of the PCB 110 is encapsulated with thermosetting resin to protect the semiconductor chip 130 and the circuit patterns 123. This encapsulant results in the formation of a package body 136. The solder balls are attached to the solder ball pads 126 on the lower surface of the PCB 110 to thereby form the solder bumps 128.
FIG. 2A depicts a step of a screen printing method in which the PCB is coated with flux by using a metal mask. FIG. 2B depicts a step of attaching the solder balls to the flux.
With reference to these figures, a screen printing method for forming the solder bumps 128 will now be described. Generally, after the PCB 110 is turned over so that the lower surface on which the solder ball pads 126 are formed faces upwards, the solder balls 128 are attached to the solder ball pads 126. More specifically, a metal mask 150 in which holes 154 are formed in a pattern corresponding to that of the solder ball pads 126 is placed on the PCB 110. Then, flux 140 is supplied onto the metal mask 150 and is forced through the holes 154 using a squeegee 156. Next, solder supplied to the metal mask 150 forms solder balls which attach to the flux 140.
The solder bumps 128 are produced by using a reflow soldering process, which is carried out under a temperature of 230.degree. C. or more. In this process, solder balls are attached to the solder ball pads 126.
After the reflow soldering process is carried out, the residue of the flux 140 which remains around the solder bumps 128 may contaminate the PCB 110, and disrupt the subsequent manufacturing processes. Therefore, it is necessary to remove the residue of the flux 140 with an organic solvent. Note, the main component of the flux 140 is a rosin.
As the number of the solder ball pads 126 increases, the pitch between the solder ball pads becomes smaller, and it becomes accordingly more difficult to align the solder balls exactly with the solder ball pads 126 using the metal mask 150. Furthermore, because the solder balls are attached to the solder ball pads 126 by the reflow soldering process, it is difficult to produce solder bumps 128 having a uniform height.
In addition, the organic solvent, which is used for removing the rosin component of the flux 140, is harmful to the environment. Another problem is that the small pitch between the solder ball pads 126 is oftentimes responsible for failures, such as shorts between adjacent solder balls. In other words, the adjacent solder bumps 128 adhere to each other causing a short between the solder bumps 128.
Also, among the semiconductor device packages which use the solder bumps as electrical connections, a micro BGA package (hereinafter, referred to as a `.mu.-BGA package`) developed by Tessera Co. (U.S.) has a problem in that the small size and pitch of the via holes of a polyimide tape, which is attached to the solder balls, can cause misalignment of the solder balls. Note also that the ratio of the height of the solder bumps to the thickness of the .mu.-BGA package is very large, that is, the height of the solder bumps is 300.about.350 .mu.m for a .mu.-BGA package having a thickness of 784.about.847 .mu.m.